Electric vehicles are increasingly practical and popular, providing users with a cleaner alternative to internal combustion driven vehicles, while also providing a degree of protection against volatile fuel costs. As used herein, the term “electric vehicle” refers to a vehicle that is propelled by one or more electric motors linked to the vehicle drive wheel or wheels, wherein the electric energy used to power the motors is stored in an onboard battery system. While an onboard auxiliary power unit, e.g., a gasoline or diesel engine, may supply additional electrical energy to the onboard battery system, the vehicle also provides a plug-in interface or other power conductive or inductive interface to allow the user to charge the onboard battery system from a municipal electrical power grid.
In theory, it would be desirable to have an onboard battery system of unlimited energy storage capacity. However in practice, the onboard battery system will typically have an energy storage capacity that is limited by weight, cost and/or size constraints. High-capacity cells in a battery of a suitable size may be prohibitively expensive, while normal capacity cells in a battery having a high overall capacity may be unsuitable in size and/or weight. For example, lead acid batteries are inexpensive, but are heavy and bulky for a given capacity, while lithium polymer and lithium ion batteries have good energy density but are constrained by cost.
Thus, other than for extended range vehicles, the practical electric vehicle will often have a range-per-charge that is less than the range-per-tank attainable with a counterpart fuel-powered vehicle. As such, it is generally important to maintain the charge state in the battery system of the electric vehicle so that the existing capacity is efficiently utilized. However, a user may not necessarily remember to plug in their vehicle after use at home or during a long stop over at a charging location. As a result, the already limited range of the vehicle may be further compromised. Thus, a system of providing battery charge state clarity and predictability is needed.